1. Field of the Invention
The present invention relates to a portable device and a battery pack capable of being used for the portable device, more specifically to a portable device having three terminals through which the portable device is electronically connected to a battery pack and the battery pack capable of being used for the portable device.
2. Description of the Related Art
Recently, more and more lithium-ion batteries have been used as secondary batteries in portable devices such as digital cameras and cellular phones. It is known, however, that lithium-ion batteries are especially susceptible to overcharging and overdischarging. Therefore, lithium-ion batteries are typically installed in a battery pack including a protection circuit against overcharging and overdischarging. Further, some battery packs having such protection circuits further include a thermistor. By having the thermistor, it becomes possible for a portable device connected to the battery pack to detect a voltage of the battery pack changing in accordance with the change of the temperature of the battery pack, so that the portable device may control the charging of the battery pack.
FIG. 1 is a block diagram showing a configuration of a conventional battery pack and a charge control section of a conventional portable device. As shown in FIG. 1, in the battery pack 1, a lithium-ion battery 2 is connected in parallel with a series circuit comprising a resistor R1 and a capacitor C1. The positive electrode of the lithium-ion battery 2 is connected to an external terminal 3 of the battery pack 1, and the negative electrode of the lithium-ion battery 2 is connected to an external terminal 4 of the battery pack 1 through n-channel MOS (Metal Oxide Semiconductor) transistors M1 and M2 capable of interrupting the current flowing through the MOS transistors M1 and M2, respectively.
The drains of the MOS transistors M1 and M2 are connected to each other. The source of the MOS transistor M1 is connected to the negative electrode of the lithium-ion battery 2, and on the other hand, the source of the MOS transistor M2 is connected to the external terminal 4. Further, body diodes D1 and D2 are equivalently connected between the drain and the source of the MOS transistors M1 and M2, respectively.
The battery pack 1 further includes a protection IC (Integrated Circuit) 5 having an overcharging detection circuit, an overdischarging detection circuit, and an overcurrent detection circuit. The protection IC 5 operates while voltages VDD and VSS are supplied to the protection IC 5, the voltage VDD being supplied from the positive electrode of the lithium-ion battery 2 through the resistor R1, and the voltage VSS being supplied from the negative electrode of the lithium-ion battery 2.
When the overdischarging detection circuit or the overcurrent detection circuit of the protection IC 5 detects the overdischarging or the overcurrent, respectively, the DOUT (output) terminal of the protection IC 5 is set to a low level to turn OFF the MOS transistor M1. On the other hand, when the overcharging detection circuit of the protection IC 5 detects the overcharging, the COUT (output) terminal of the protection IC 5 is set to a low level to turn OFF the MOS transistor M2. The battery pack 1 further includes a thermistor R3 connected between the external terminal 4 and an external terminal 6 of the battery pack 1.
While the lithium-ion battery 2 is being charged, a predetermined voltage is present at the external terminal 6 through a divided-voltage resistor R4 by a reference voltage source Vref1 11 of a portable device 7 connected to the battery pack 1. The resistance of the thermistor R3 changes in accordance with the change of the temperature of the battery pack 1. Therefore, the voltage at the external terminal 6 changes in accordance with the change of the temperature of the battery pack 1.
The portable device 7 includes terminals 8, 9, and 10 electronically connectable to the terminals 3, 4, and 6, respectively, of the battery pack 1 when the portable device 7 and the battery pack 1 are connected to each other. The terminals 8 and 9 are a positive power source terminal and a negative power source terminal, respectively. The terminal 10 is used for detecting the voltage at the external terminal 6. Further, the portable device 7 includes the reference voltage source Vref1 11, a current source 12, a diode D3, a comparator 13, a charging control circuit 14, and a MOS transistor M3.
The voltage at the external terminal 6 and accordingly at the terminal 10 is input to one of the input terminals of the comparator 13. The voltage at the terminal 10 is the divided voltage calculated based on the values of the reference voltage 11, the resistor R4 and the thermistor R3. A predetermined voltage VT generated by the current source 12 and the diode D3 is input to the other input of the comparator 13. The output of the comparator 13 changes when the voltage detected by the terminal 10 is lower than the predetermined voltage VT. The output of the comparator 13 is input to the charging control circuit 14.
The charging control circuit 14 turns ON and OFF the MOS transistor M3 in accordance with, for example, the charging current or the charging voltage. Further, when the external terminals 3 and 4 of the battery pack 1 are electrically connected to the terminals 8 and 9, respectively, the charging control circuit 14 becomes capable of detecting the increase of the temperature of the battery pack 1 by detecting the change of the voltage at the terminal 10. When the increase of the temperature of the battery pack 1 is detected, the charging control circuit 14 prevents the battery pack 1 from being charged. More specifically, when the resistance of the thermistor R3 is reduced and, as a result, the voltage detected at the terminal 10 is lower than the predetermined voltage VT, the charging control circuit 14 is deactivated to turn OFF the MOS transistor M3 to prevent battery pack 1 from being charged.
In such a configuration including a conventional battery pack and a conventional portable device connected to the battery pack, there may be also included a functional circuit such as a status detection circuit capable of detecting a status such as remaining capacity of the lithium-ion battery 2. In such a case, a communication terminal to transmit the output signal from the status detection circuit to the portable device is additionally provided on the battery pack. Through the communication terminal of the battery pack, the portable device connected to the battery pack receives the signal indicating the status information of the battery pack in order to control the status of the battery pack.
Japanese Patent Application Publication No. 2000-209788 discloses a charging device capable of receiving a control signal through the communication terminal of a battery pack so that the received control signal may be used for controlling the charging of the battery pack.
However, when the communication terminal is additionally provided in a conventional battery pack, the number of terminals of the battery pack is increased to four (4). If this is the case, the portable device is also required to have four terminals, which necessitates a change in the specifications of the portable device, which may cause a cost increase. Further, from a physical point of view, all the additional parts due to the specification change may also be required to be installed in the same limited space inside the battery pack. Therefore, it may be difficult to provide the additional terminal in the battery pack.